image value from yaml gets expanded to fully qualified name, breaking local backend

[thestr4ng3r]

Component

server

Describe the bug

When parsing the image in a pipeline yaml, the given value is always expanded to a fully qualified docker image name very early:

woodpecker/pipeline/frontend/yaml/compiler/convert.go

Line 23 in 3f73d5b

image = expandImage(container.Image)

For example, if you have image: alpine, you will get docker.io/library/alpine:latest.

First, it is questionable whether this kind of expansion should be done on the server at all, as the docker or podman on the agent machine could figure this out themselves, as @6543 noted on matrix.

But second, this is a major problem for the local backend where the image value is not interpreted as a docker image, but as the name of the shell to run commands in.
Currently there is this hack in place, which tries to chop off the prefix and suffix again:

woodpecker/pipeline/backend/local/local.go

Line 65 in 3f73d5b

Command = append(Command, proc.Image[18:len(proc.Image)-7])

So you pass image: bash and proc.Image[18:len(proc.Image)-7] will evaluate to bash. But this of course is only a very fragile workaround. For example it panics and crashes the agent as soon as you pass something like image: /bin/bash, which will not get anything added by the expansion.
This by itself could also be fixed by not reusing the image key for the shell and instead adding an additional one.

System Info

3f73d5b

Additional context

No response

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[6543]

☝️ race condition 😆